The present invention relates to a method and a computer program product for optimizing the production of paper or board.
In this description, the term “paper” or “board” refers to web-like products, which are formed at least mainly from a stock refined from vegetable fibre. The most important products in the group are printing papers, packing boards, and tissue papers. In addition, various special products and combination products are manufactured, in which case artificial polymer films, metal foils, or fillers can be added to the fibre-stock web. For the sake of simplicity, only the manufacturing processes for printing papers will be examined in the following, but the principles of the invention can also be applied to the other products referred to above.
The manufacturing process begins with a fibre stock mixed with water being prepared and spread onto a wire. Spreading takes place using a head-box, which forms an even layer of stock on the wire. The raw materials used in manufacture, such as cellulose pulp, can be made in the same mill, or purchased from an external manufacturer. The properties of the stock depend on its manufacturing method and raw materials, mixtures of different raw materials being frequently used. The manufacture of paper or board uses, among other materials, mechanical fibre, cellulose pulp, recycled fibre, broke returned to the process, and a wide variety of chemicals, binders, and colouring agents. The properties of the fibre stock used greatly affect the properties and price of the paper. By using different stocks, the strength, brightness, and surface quality of the paper, for instance, can be influenced. When manufacturing a specific type of paper grade, an attempt is usually made to select a combination of the most economically priced raw materials that can be used to achieve the quality targets set by the customer. The price of the base paper is particularly strongly affected by the type of the stock, or the combination of stocks. As the prices of fibre pulps vary continuously, it is advantageous to alter the stocks according to the price variations, in order to achieve the best possible profit. If recycled stock is used, the selected stock composition may be affected by even the availability of recycled fibre. If enough recycled fibre is not available, it must be replaced with other pulp. Because the quality and properties of the base paper have a considerable effect on the following stages of the manufacturing process, it is difficult to optimize the manufacture of the base paper.
Though a great amount of water leaves the base paper stock on the wire, it must still be effectively dried. Drying takes place by pressing between felts and finally with the aid of heat, usually on dryer rolls. By regulating and dividing the drying event, the removal of water from the web in different stages can be used to affect the properties of the paper and particularly the amount of energy required. After drying, the paper may be complete, or it can be taken for further processing, either to devices that are a direct continuation of the paper machine, or to separate finishing devices. When manufacturing printing papers, the devices used for finishing are usually calenders for improving the smoothness of the paper, and various coaters for applying different substances to the surface of the paper. Usually pigment coatings, such as kaolin and calcium carbonate, are used. There is a great number of different coating agents and combining them permits a nearly unlimited number of different coating mixtures to be created. In addition to the above methods, paper is treated with surface size to increase its strength and with various additives, which are added, for example, to the pulp.
All the manufacturing stages referred to above affect the properties of the paper that is created as the end product. For example, if a darker pulp is used, a greater amount of coating will be required to achieve the same surface whiteness. Similarly, if a more expensive long-fibre stock is used, a greater proportion of the stock can be replaced with a filler, so that the amount of stock can be reduced. In both of the above cases, the most economical mix ratio is determined by the purchase price of the production goods, while the boundary values of the available range of variation are determined according to the desired properties of the final product. Thus, the limits within which a final product meeting the quality requirements can be manufactured also limit the variations in the amounts and proportions of the raw materials. Besides energy and raw materials, the production factors relating to the costs of the product include tangible and intangible capital, personnel resources, and various services, such as transport, maintenance, cleaning, marketing, security, and the purchasing of external expertise. The production factors must be either purchased from outside the company, or produced within it.
If only two variables are examined, as above, a change in the process and its effects can be easily understood. If it is considered that each change made in the process will have many different effects on the properties of the paper, the costs of manufacture, the runnability of the machine, and the amounts of raw materials required in manufacture, it will become obvious that the effect of even a single change is difficult to assess. The management of the process is made even more difficult by the fact that individual process changes must be compensated by correspondingly altering other process factors. Thus, it is easy to become involved in a chain of changes that is difficult to predict.
In paper machines, highly developed control system are used, which monitor the quality and properties of the product being manufactured. However, these control systems usually control only a single variable property, such as the moisture content of the paper in different stages of manufacture, or the thickness or thickness profile of the paper. Thus, for example, a change in moisture content in an early process stage will affect the quality of the base paper in terms of whether the paper requires wetting or drying prior to calendering in a later process stage. Usually, there are still wetting and drying devices at the calender, by means of which the moisture content of the paper is suitably adjusted for calendering. In addition, the calender has its own moisture control system. This system is intended to keep the moisture within the set limits, but is in no way able to take into account, for example, wetting or drying requirements arising from changes in the fibre quality. Correspondingly, the other control processes operate relatively independently and attempt to keep the variable they control within the set limits. Thus, the control system cannot react, for example, to adjustment requirements arising from changes in raw materials, except in the case of its own variable. Due to this, in a state of change, the machine operator must set the necessary running values in the control system, after which the control must usually be fine tuned to achieve the final properties of the product. This control setting is based on previous production data, with the aid of the experientially based know-how of the machine operators.
In the paper manufacturing process and its sub-processes, such as the manufacture of the base paper, the operator must control several management variables. These include speed, temperature, the linear load in the calender, wetting, the coating amount and mix, the drying effect and its distribution through the different drying stages, the raw materials and their proportions, and so on. In addition to these, the prices of energy and raw materials, which change continuously, must also be taken into account. The choices made by the operator concerning these variables affect, in a manner that is, however, generally predictable through theory and experience, the various properties of the product being manufactured, as well as its price.
A paper maker's objective is to manufacture as cost-effectively as possible paper that meets the customer's quality requirements. For this purpose, measurement methods have been developed, which are intended to be used to predict the behaviour of the paper in a printing press and as an end product. Correspondingly, measurement methods for the properties of other products have also been developed. Information on many properties that affect the quality of paper are obtained already on the paper machine, with the aid of online measurements. In addition, traditional laboratory measurements and monitoring is used, along with the possibility to use automated paper testing laboratories. These data can be processed and transferred in an electronic form.
From the point of view of the user of printing paper, the most important aspect is the final result of the printing. The printing result is affected by the runnability of the paper, i.e. the number of printed products produced in a unit of time, a high running speed, printability, i.e. the amount of ink required to achieve the quality characteristics, and the information carrying capacity, which consists of the darkness of the impression, the capacity to reproduce details, and the unevenness of the impression. A manufacturer of printed products must also take into account the price of printing, which is strongly affected by the paper. The higher the desired quality of the impression, the more highly processed and expensive the paper that must be used, thus raising costs. It is therefore most economical for the paper buyer to purchase the cheapest paper that meets the quality requirements.
The control variables of the manufacturing process often affect in an individual manner nearly all the properties being measured. The criteria set for the properties of the product can be met using several different combinations of the control variables. The search for the best values, and often for those optimized other than directly in terms of the properties of the product (such as the amount of energy used in different forms), demands knowledge, experience, and time. The user interfaces of the measurement results depicting the properties of the product and of the control variables affecting the process do not meet the requirements related to making such examinations. Usually, the information is dispersed very widely over different systems and it is laborious to gain an overall picture. For example, forming a real-time picture of the runnability, printability, and information carrying capacity of the paper being made on a paper machine is impossible, as is the real-time control of the paper machine on the basis of these properties.
In principle, multi-variable control implemented in the correct manner could form a tool for improving manufacture, but it is neither economically nor technically feasible to implement such a system with the aid of the present level of technology. A second weakness relating to control systems is that the control always functions according to a control strategy built into the control system. This control strategy is always based on models that have previously been shown to be good, in which limits are set, within which the control must remain. Thus, it may not necessarily be possible to find optimal values for complex systems, if, for example, the adjustment range of some variable must be exceeded, but the exceeding can be compensated for with the aid of other control variables.